1. Field of the Invention
The present invention relates to a tensioner for an endless transmission belt and an internal combustion engine including the tensioner.
2. Discussion of the Background
A hydraulic tensioner is a tension-maintaining mechanism having a basic principle of pressing a plunger to an endless transmission belt by means of a spring and the hydraulic pressure. Various hydraulic tensioners of different structures have been proposed thus far. See, for example, Japanese Patent Application Publication No. 2006-144999. The contents of Japanese Patent Application Publication No. 2006-144999 are incorporated herein by reference in their entirety.
In FIG. 2 of Japanese Patent Application Publication No. 2006-144999, a hydraulic tensioner (28) (Note that the reference numeral in parentheses is one used in the Japanese Patent Application Publication. The same notation is employed in the following description.) includes a plunger body (32), a plunger (31) which is movably installed in a circular hole (34) formed in the plunger body (32), a compression coil spring (50) that biases the plunger (31), a plunger-side hydraulic chamber (45) that stores hydraulic oil used for pushing out the plunger (31). The hydraulic tensioner (28) is a mechanism for giving a tensile force to a timing chain (4) by pushing out the plunger (31) by means of both the act of the spring and the act of the hydraulic pressure.
A through hole (35) that has an opening in a chain chamber (24) is formed in the head portion of the plunger (31), and a ball check valve (36) that closes the through hole (35) is also provided in the head portion. The ball check valve (36) includes a ball (37) and a compression coil spring (38). When the plunger (31) is excessively pushed leftward in the figure, the internal pressure of the plunger-side hydraulic chamber (45) increases beyond a predetermined pressure and, therefore the ball (37) is moved rightward in the figure against the force applied by the compression coil spring (38). Then, the hydraulic oil in the plunger-side hydraulic chamber (45) is discharged via the through hole (35) to the chain chamber (24), so that the internal pressure of the plunger-side hydraulic chamber (45) is decreased. With the decrease in the internal pressure, the ball (37) moves leftward in the figure, so as to stop the discharging of the hydraulic oil.
As has been described thus far, the ball check valve (36) is a valve which prevents the excessive increase in the internal pressure of the plunger-side hydraulic chamber (45) by discharging the hydraulic oil, and the valve of this type is called a pressure relief valve, in short, a relief valve. So, hereafter, the ball check valve (36) is referred to as the relief valve (36).
Now, suppose a case where the internal pressure of the plunger-side hydraulic chamber (45) exceeds the predetermined pressure, and the hydraulic oil in the plunger-side hydraulic chamber (45) is discharged to the chain chamber (24). In this case, the hydraulic oil in the plunger-side hydraulic chamber (45) decreases by the amount of the discharged hydraulic oil so that the pressure is decreased. Accordingly, it is necessary to supply the hydraulic oil to the plunger-side hydraulic chamber (45) by an oil pump via an oil passage hole 62. The recovering of the level of pressure calls for a quick supply of the hydraulic oil. To meet the needs of such a quick supply, an oil pump of higher capacity is required. As a consequence, the oil pump to be employed for the purpose has to be larger in size.
Meanwhile, the stopping of the engine may cause a phenomenon of a reverse rotation that the crankshaft slightly rotates in a reverse direction immediately before the crankshaft stops. The reverse rotation of the crankshaft tightens the side of the timing chain (4) that has been loose thus far. The tightening strongly pushes back the plunger (31), opens the relief valve (36), and makes the hydraulic oil in the plunger-side hydraulic chamber (45) flow out to the chain chamber (24). At this time, the crankshaft is not moving, and thus the oil pump is not in operation. Consequently, the plunger-side hydraulic chamber (45) is not to be refilled with the hydraulic oil. As a consequence, the timing chain (4) is kept in a state that is opposite to the ordinary state. To put it differently, the side of the timing chain (4) that has been tight thus far is maintained in a loose state. The loose side of the timing chain (4) may produce a noise when the engine starts next time.